Rheometer with angled blades

ABSTRACT

A rheometer for assessing characteristics of materials includes a vessel for containment of a material to be assessed, and a rotor which is, in use, passed through the material contained in the vessel, the rotor being comprised of a plurality of blades disposed at an angle relative to the axis of the rotor shaft. The vessel and rotor are constructed and adapted for rotation relative to one another about an axis and also for relative movement in the axial direction, and least one transducer is provided for determining the rotational and (optionally) axial forces that result from the relative motions(s) of the rotor and the material in order to thereby assess the characteristics of the material.

The present invention is concerned with a rheometer for assessingcharacteristics, such as flow characteristics, of materials.

Rheometers are used in a wide variety of chemical and materialprocessing industries including pharmaceuticals, food processing,agrochemicals, paint and pigment manufacture, paper manufacture,catalysts, ceramics and cosmetics for determining and comparingcharacteristics, such as flow characteristics, of materials such aspowders, liquids and semi-solids such as pastes, gels, ointments and thelike. These materials generally combine the properties of a viscousliquid and an elastic solid, that is they are visco-elastic. Theresponse of such materials to mechanical force is important for theirproper manufacture and use and as a result rheometry is an importanttool in their development, production and quality control.

Viscometers and rheometers are well known for assessing thecharacteristics of liquids and some semi-solids. However, these knowninstruments have limitations and are not suitable for many kinds ofsemi-solid materials. In particular, they are not suited to assessingthe characteristics of powders when or while being mixed with othermaterials such as solid or liquid binders, surfactants or air. Knowntwin rotor mixing machines have been modified to carry out thisassessment and a number of "mixer torque rheometers" are availablecommercially. These apparatus employ complex rotors, have a poorvolumetric efficiency and lack sensitivity and repeatability. The lackof sensitivity and repeatability are due to the dependence of theseapparatus on material being squeezed between two rotor blades rotatingat different speeds.

The torque transient associated with this squeezing is recorded and usedto assess the flow characteristics of the material. However, the amountof material trapped between the rotor blades is not consistent and leadsto variable results. Only the speed of the rotors can be varied so thatthere is limited scope for sensitively testing materials having widelydifferent characteristics.

Some torque rheometers have rotors that do not overlap, and the torquerequired to rotate one of the counter-rotating shafts is used tocharacterise the material. However, this type of torque rheometer haspoor sensitivity and requires significant amounts of material in orderto complete a test.

The poor volumetric efficiency of the known apparatus can lead to asignificant proportion of the material remaining unmixed and this is afurther cause of variability in the results. Additionally, poorvolumetric efficiency renders the apparatus unsuited for assessing smallquantities of material which is an important requirement for drugformulation.

A further problem with known apparatus is the difficulty of cleaning thebowl and rotors after use. Cleaning is important to avoidcross-contamination and in some cases to ensure recovery of as muchmaterial as possible.

U.S. Pat. No. 5,118,439 describes a viscometer in conjunction with ahelipath stand in which a rotating shearing spindle describes a helicalpath through a test sample.

GB-A-2 092 308 describes a paddle arrangement for measuring theworkability of concrete. The paddle is rotated in the concrete and thetorque required to turn the paddle when immersed in fresh concrete ismeasured.

U.S. Pat. No. 4,530,701 describes the use of a Brookfield viscometerwith helipath attachment in which spindles in the form of wire teestraced out helical patterns through fresh, unsheared volumes of sample.WO-A-9203719 describes a rheometer in which a cylindrical spindle isrotated in a cylindrical sample chamber. U.S. Pat. No. 3,935,729describes a coaxial cylinder rheometer in which a cylindrical rotorrotates within an outer cylinder which contains a material to be tested.The rotor is movable axially as well as rotationally. WO-A-9509353describes a viscometer in which the rotor can be changed automatically.

It is therefore an object of the present invention to provide arheometer which eliminates or at least ameliorates the above-identifiedproblems.

According to the present invention there is provided a rheometer forassessing characteristics of materials, the rheometer comprising:

a vessel for containing a material, the characteristics of which are tobe assessed;

means disposed in use within the vessel for passing through a materialto be assessed; and

the vessel and the means disposed therewith in being constructed andadapted so as to be simultaneously:

rotatable relative to each other about an axis, and

movable in the axial direction relative to each other,

wherein means is provided for determining rotational forces as a resultof said relative motion in order to assess the characteristics ofmaterial within the vessel and wherein the means disposed within thevessel comprises rotor means in the form of a plurality of bladesextending substantially radially from a rotor shaft and disposed at anangle relative to the axis of the shaft.

The rheometer may include means for determining axial forces as a resultof said relative motion in order to assess the characteristics ofmaterial within the vessel.

Suitable materials can include liquids, powders and semi-solid mixturesof liquids and powders. The materials may additionally include airand/or one or more other gases.

The rheometer can be employed with individual ingredients orformulations that have previously been mixed using other means, forexample as part of a production process. The rheometer is also capableof mixing ingredients together.

Control and measurement may be effected both during mixing and testing.

The rheometer can be used as a free-standing instrument or can beincorporated as an on-line unit built into product production equipment.

An important aspect of the rheometer according to the present inventionis that passage through the material of the means for passing throughthe material to be assessed during assessment of the flowcharacteristics of the material is steady state. This gives rise to highrepeatability of test results both on a given rheometer according to thepresent invention and between different rheometers according to thepresent invention.

By "steady state" there is meant herein that the set of variablesdefining the passage through the material to be assessed are eitherconstant or varying at a relatively slow rate. "Relatively slow" in thepresent case means that the rheometer is capable of establishing adesired flow pattern that can be maintained for sufficient time (forexample, at least several seconds) for the characterising data to becollected. In contrast to this, known mixer type rheometers rely oncollecting transient data where the patterns of flow are constantlychanging in a complex manner.

The vessel and the means disposed therewith in may be rotatable relativeto each other at variable speed.

The vessel may be cylindrical or tapered, for example with therelatively narrow region of the vessel at the lower end thereof. Thevessel may be made of a transparent material. The vessel may bereciprocable in the axial direction and may be moved at variable speedif desired. The vessel may be mounted on a table, for example by meansof a clamp.

Alternatively or additionally, the means disposed within the vessel maybe reciprocable in the axial direction and may be moved at variablespeed if desired.

The means disposed within the vessel may pass through the material to beassessed in a manner such as to displace, move or work the materialwithin the vessel. The forces imposed on the material during thisprocess may be controlled so as to avoid over-compaction of the materialand/or modification of the characteristics of the material.

The blades may be of twisted form, for example the angle of twist may bein proportion to the radial dimension of the blade. The rotor means maybe rotatable clockwise and anti-clockwise. If desired, the rheometer mayinclude means for holding the rotor stationary.

The combined motion of the vessel and the rotor means may be such thatthe rotor means describes a predetermined path through the material,usually helical in form although other controlled paths, such ascircular and variable pitch helical paths, are possible. The relativeaxial and rotational speeds may determine the helix angle, the amount ofmaterial displaced and the forces imparted to the material duringrelative movement.

The determining means may include means for determining rotational (andoptionally axial) forces on the means for passing through the materialand on the vessel. The determining means may include controlling means.

The means for determining axial forces may comprise a force transducer.The means for determining rotational forces may comprise a torquetransducer. The difference between rotor torque and vessel torque may bedetermined where required by using two torque transducers. The energyinput to the material is readily monitored or controlled or limited byreference to the relative speeds of the vessel and the means therewithin together with the axial and rotational forces.

The rheometer may be employed additionally to collect or compactmaterial prior to assessment. Compaction may be limited to apredetermined level.

The rheometer may be employed additionally to mix constituents of thematerial prior to assessment. One or more constituents of the materialmay be added incrementally.

For a better understanding of the present invention and to show moreclearly how it may be carried into effect reference will now be made, byway of example, to the accompanying drawings in which:

FIG. 1 is a diagrammatic illustration of one embodiment of a rheometeraccording to the present invention;

FIGS. 2(a)-(d) are a diagrammatic illustration of various assessmentmodes available with the rheometer according to the present invention;

FIGS. 3(a)-(f) are a diagrammatic illustration of various mixing modesavailable with the rheometer according to the present invention; and

FIG. 4 is a diagrammatic illustration of another embodiment of arheometer according to the present invention.

The rheometer shown in FIG. 1 comprises a generally cylindrical vessel 5for containing a material 7 to be assessed and a rotor 6 provided with apair of radial blades 15 which are angled relative to the axialdirection and which are a close fit within at least a part of thevessel. The rotor 6 is substantially coaxial with the vessel 5 and maybe held stationary or may be rotated at a variable speed eitherclockwise or anti-clockwise by a variable speed drive 1, such as aservo-motor, by way of a gearbox 2. The variable speed drive may becapable of functioning as part of a closed loop system so thatpredetermined levels or rates of change of force or torque may beachieved as will be described in more detail hereinafter. The vessel isprovided with a scraper bar 11 in the region of the upper inner edgethereof to minimise any loss of material during use of the rheometerwhen the vessel is in the region of its fully lowered position. Thevessel may be made of PYREX or similar transparent material to allowvisual observation of the mixing and assessment process.

However, the vessel may be made of other materials or may take otherforms if desired. For example, the vessel may be made of metal such asstainless steel and/or the vessel may be enclosed within a heatingjacket for the preheating of material prior to and/or during assessment.The vessel may be closed at the upper end thereof if desired, theclosure being provided with a suitable seal for the passage of the rotorshaft: such an arrangement allows collection of material at the top ofthe vessel followed by assessment in this position, allowing thematerial to be sheared and to fall to the bottom of the vessel undergravity such that sheared material cannot interfere with the remainderof the assessment procedure. The vessel may be tapered to providevariable clearance between the periphery of the rotor and the wall ofthe vessel: such an arrangement facilitates investigation of "powderpacking" in which higher than normal resistance to flow is found whendisplacing material in close proximity to the wall of a containingvessel. Material exhibiting exothermic properties may be assessed in avessel having suitable low thermal mass characteristics and a rotorshaft of low thermal conductivity. A temperature sensor may be providedto permit thermal characteristics to be studied. The relative isolationand simplicity of the vessel and rotor allow for the safe assessment ofdangerous materials such as radioactive materials. A zone enclosing thevessel can be surrounded by appropriate screening and the assessment canbe carried out automatically. Contaminated components, such as thevessel and possibly the rotor, can be disposed of, which is lesspractical with known mixer type apparatus.

The rotor assembly may be a single rotor having twin blades asillustrated, but alternatively the rotor assembly may incorporate twinrotors arranged coaxially such that each rotor may be held stationary ormay be rotated in either direction irrespective of the direction ofrotation of the other rotor so as to provide a wide range ofdifferential rotor speeds. Twin blades may be provided as shown, forexample of standard diameters adapted to different vessel diameters, butalternatively the or each rotor may have more that two blades, forexample four, or special blade shapes may be employed.

The size of the rotor and the vessel may be selected to suit the amountof material to be assessed. For the pharmaceutical industry, the amountof material may range, for example, from 3 to 1000 grams. The ability toassess a small amount of material is especially important for thedevelopment of specialist drugs in the pharmaceutical industry.

The vessel 5 is supported on a reciprocating table 8 which can be raisedor lowered in the axial direction of the vessel by means of a linearguidance system 12 the construction of which is well known to theskilled person, the vessel being secured to the table by a clamp 14. Thetable 8 is itself supported on a low friction bearing 13. The linearguidance system can be operated at a variable velocity by means of avariable speed reversible drive 10 and a gearbox 9.

The combined movements of the rotor and the vessel cause the rotor tomove along a helical path through the material 7 contained within thevessel. The condition of least resistance to movement will occur whenthe helix angle is equal to the blade angle of the rotor. conversely, itis possible to achieve a combination of speeds of the rotor and thevessel such that the direction of movement of the blade through thematerial 7 is substantially perpendicular to the blade face. In thiscase, the resistance to movement will be a maximum and a maximum amountof material will be displaced. It will be clear that variation of thecombination of speeds of the rotor and the vessel can additionally giverise to a complete range of controlled movement of the rotor bladesrelative to the material 7. Examples of operating modes will bedescribed hereinafter with reference to FIGS. 2 and 3.

A force transducer 4 is provided to measure axial forces (compressionand/or tension) imposed on the material 7 as it is displaced and atorque transducer 3 is provided to measure torque imposed on thematerial. The force transducer can be positioned to measure axial forcesin the support for the table 8 as shown in FIG. 1 and/or in the shaftfor driving the rotor 6. The torque transducer can be attached to thetable 8 as shown in FIG. 1 and/or can measure the torque applied to theshaft for driving the rotor 6. The transducers 3 and 4 can additionallybe used to prevent overload.

The rheometer is controlled by a computer (not shown) which monitorsrotor force and torque and controls the speed of the rotor and tabledrives. Depending on the relative speeds and directions of the drives,various assessment modes are available, including:

(a) tests using predetermined combinations of rotor and table speeds;

(b) tests limited by predetermined force or torque settings; and

(c) constant shear stress testing whereby a fixed or programmed rate ofchange rotor torque or rotor force is maintained.

FIG. 2 shows various assessment modes with the arrow in each drawingindicating the direction of movement of the blade relative to thematerial within the vessel. FIG. 2(a) illustrates the situation wherethe helix angle is equal to the blade angle of the rotor (6), that isthe condition of least resistance to movement of the blade and minimumdisplacement of the material. FIG. 2(b) illustrates an intermediatecondition of moderate resistance to movement and moderate displacementof material. FIG. 2(c) illustrates the situation where the direction ofmovement of the blade is substantially perpendicular to the blade face,that is the condition of maximum resistance to movement of the blade andmaximum displacement of the material. FIG. 2(d) illustrates the use ofthe apparatus to assess the forces required to shear the materialadhering to the wall of the vessel.

The rheometer according to the present invention can be used in at leasttwo different ways. In the case of powders and materials that havepreviously been prepared or mixed, the rheometer can be used to assessflow properties of the material by testing the material in one of theassessment modes described above. In order to ensure that the materialis first suitably collected in the bottom of the vessel in preparationfor testing, the rotor may be driven while the vessel is being raised ina direction to push the material in the opposite direction to thedirection of movement of the vessel (FIG. 3(a)). The amount ofcompression or squeezing of the material when collected in this way canbe limited by sensing the force and/or torque levels to ensure that apredetermined maximum is not exceeded. The limitation of force and/ortorque levels is useful where it is important to avoid over-compactionof the material. For example, compression stresses may be limited byreference to processing and testing parameter information available tothe controlling computer.

Some materials, however, will require a number of different constituentsto be added to the vessel and mixed either prior to or during assessmentof the flow characteristics of the material. The rheometer according tothe present invention is capable of processing, or mixing theconstituents of the material by utilising appropriate mixing modes. Forexample, a stiff, dough-like material may require to be chopped prior tobeing compressed, whereas a powder may need to be aerated and notcompacted. FIG. 3 shows various mixing modes with the arrow in eachdrawing indicating the direction of movement of the blade relative tothe material within the vessel. FIG. 3(a) illustrates the situationwhere the material is being collected in the bottom of the vessel inpreparation for testing. FIG. 3(b) illustrates the converse of FIG. 3(a)in which the material is being collected at the top of the vessel. FIG.3(c) illustrates slicing and downwards displacement of materialcollected at the top of the vessel. FIG. 3(d) illustrates a mixing modein which material is sliced and projected against the wall of thevessel. FIG. 3(e) illustrates a mixing mode in which the movement of theblade through the material is substantially perpendicular to the bladeface, that is maximum displacement of material. FIG. 3(f) illustrates amixing mode in which the blade angle is substantially equal to the helixangle and the blade slices through the material with minimumdisturbance.

Mixing and/or assessment conditions may also be determined by referenceto information available to the controlling computer including, forexample, details of test programmes. For example, a wet mix could bemixed and assessed while a binder (such as water) is incrementally orprogrammably added. A mixing and testing programme would then run untilpredetermined criteria are achieved.

Programmable binder addition may be effected, for example at a givenvolume per unit time, while the vessel is traversing downwardly orupwardly, or when the vessel is fully raised (i.e. the rotor is at thebottom of the vessel), and improves the distribution of the binderthroughout the material and therefore affects the consistency of thematerial and the accuracy of the assessment. The binder may be added byinjection into the vessel or by way of a rotor having a hollow shaftthrough which the binder can be pumped so as to be added to the materialin the region of the rotor. In either case, binder addition can beincorporated into the processing and testing programme so as to beeffected automatically.

In addition to, or as an alternative to, programmable binder addition,other constituents of the material may be added in an incremental orprogrammed manner. Such incremental or programmed additions give theoption of mixing and assessing materials having varying proportions ofconstituents. As with binder addition, incremental or programmedadditions of constituents can be effected automatically.

The amount of energy input to the material during the mixing andassessment stages can readily be calculated from the rotor speed andtorque measurements and from the rotor force and vessel speedmeasurements. The variation of energy input as, for example, a functionof time, revolutions of the rotor or excursions of the vessel can bedisplayed graphically if required.

The mixing and/or squeezing actions used are controllable so that thelevels of shearing and compaction applied can be contained withinpredetermined limits.

We have found that the rheometer according to the present invention iswell adapted for use in the following activities:

(a) Formulation, that is the formulation or development of new productsinvolving a relatively small amount of material;

(b) Quality control applications, for example using small quantities ofmaterial to assess:

b1) source variation in excipients and drugs;

b2) batch variation in excipients and drugs;

b3) binder type and concentration;

b4) production monitoring;

(c) Establishing optimum processing conditions and the effect ofprocessing variables such as mixing time;

(d) Scaling, that is deriving information from a small scale assessmentfor use in determining conditions appropriate for large scalemanufacture.

The rheometer according to the present invention has high sensitivityand is capable of assessing characteristics, such as flowcharacteristics, of powders, liquids and semi-solids having a very widerange of viscosity. This sensitivity is achieved because the angle atwhich the blades of the rotor approach the material can be varied tosuit the flow characteristics of the material itself. The flow forcesacting on a blade when moving through a material, such as a powder,depends greatly on the angle of the blade with respect to the directionof movement. At right angles, the resistance to movement is very highand relative movement may not be possible, but when in line the bladewill cut easily and experience little resistance. Assessment cantherefore be carried out at whatever "angles of approach" areappropriate for any particular material.

With the blades being a close fit within the vessel, the unswept volumewithin the vessel is small. The result of this is that the mixingefficiency of the rheometer is high in comparison with known apparatus.

The rheometer shown in FIG. 4 is similar to that shown in FIG. 1, themost significant difference being that relative axial movement iseffected by movement of the rotor rather than the vessel.

The rheometer shown in FIG. 4 comprises a generally tapered vessel 25for containing a material 27 to be assessed and a rotor 26 provided witha pair of radial blades 35 which are angled relative to the axialdirection and which are a close fit within at least a part of thevessel. The rotor 26 is substantially coaxial with the vessel 25 and maybe held stationary or may be rotated at a variable speed eitherclockwise or anti-clockwise by a variable speed drive 21, such as aservo-motor, by way of a gearbox 22. As with the embodiment of FIG. 1,the variable speed drive may be capable of functioning as part of aclosed loop so that predetermined levels or rates of change of force ortorque may be achieved. The vessel is provided with a scraper bar 31 inthe region of the upper inner edge thereof to minimise any loss ofmaterial during use of the rheometer when the rotor is in the region ofits fully raised operational position.

The form of the rotor blade and of the vessel may be substantially thesame as those described above in respect of FIG. 1.

The rotor 26 can be raised and lowered in the axial direction by meansof a linear guidance system 32 the construction of which is well knownto the skilled person. The liner guidance system 32 can be operated at apredetermined variable velocity by means of a variable speed reversibledrive 30 and a gearbox 29.

A torque transducer 23 is provided to measure torque imposed on thematerial 27. The torque transducer can be applied to the shaft fordriving the rotor 26 as shown in FIG. 4 and/or can be attached to thetable 28.

The vessel 25 is secured to a table 28 by means of a clamp 34, a forcetransducer 24 being provided to measure axial forces (compression and/ortension) imposed on the material 27 as it is displaced. The forcetransducer 24 can be positioned to measure axial forces in the supportfor the table 28 as shown in FIG. 4 and/or in the shaft for driving therotor 26.

Operation of the rheometer shown in FIG. 4 is essentially the same asthat shown in FIG. 1.

An important aspect of the rheometer according to the present inventionis that the shear flow (or compression) phase set up during assessmentof the flow characteristics of the material is steady state. This is indirect contrast to known apparatus based on conventional mixers whichrely on a transient torque peak. The steady state flow mode as the rotorfollows a helical path allows the shear flow (or compression) phase tobe maintained in a manner that is readily quantifiable in terms of forceand torque measurements and amenable to theoretical analysis.Additionally, the steady state flow mode allows the variability of thematerial to be assessed as the vessel is raised and the total volume ofthe material is displaced. The rheometer according to the presentinvention is therefore sensitive to non-homogeneity of the materialwhether this arises as a result of inadequate mixing or due to otherfactors.

We claim:
 1. A rheometer for assessing characteristics of materials, therheometer comprising:a vessel (5, 25) for containing a material (7, 27),the characteristics of which are to be assessed; means (6, 26) disposedin use within the vessel for passing through a material to be assessed;and the vessel (5, 25) and the means (6, 26) disposed therewith in beingconstructed and adapted so as to be simultaneously:rotatable relative toeach other about an axis, and movable in the axial direction relative toeach other, wherein means (3, 23) is provided for determining rotationalforces as a result of said relative motion in order to assess thecharacteristics of material (7, 27) within the vessel (5, 25) andwherein the means (6, 26) disposed within the vessel comprises rotormeans in the form of a plurality of blades extending substantiallyradially from a rotor shaft and disposed at an angle relative to theaxis of the shaft.
 2. A rheometer as claimed in claim 1, wherein means(4, 24) is provided for determining axial forces as a result of saidrelative motion in order to assess the characteristics of material (7,27) within the vessel (5, 25).
 3. A rheometer as claimed in claim 1,wherein the vessel (5, 25) and the means (6, 26) disposed therewith inare rotatable relative to each other at variable speed.
 4. A rheometeras claimed in claim 1, wherein the vessel (5, 25) is cylindrical.
 5. Arheometer as claimed in claim 1, wherein the vessel (5, 25) is tapered.6. A rheometer as claimed in claim 5, wherein the vessel (5, 25) istapered with a relatively narrow region of the vessel at a lower endthereof.
 7. A rheometer as claimed in claim 1, wherein the vessel (5,25) is reciprocable in the axial direction thereof.
 8. A rheometer asclaimed in claim 7, wherein the vessel (5, 25) is movable at variablespeed.
 9. A rheometer as claimed in claim 1, wherein the means (6, 26)disposed within the vessel is reciprocable in the axial directionthereof.
 10. A rheometer as claimed in claim 9, wherein the means (6,26) disposed within the vessel is movable at variable speed.
 11. Arheometer as claimed in claim 1, wherein the blades are of twisted form.12. A rheometer as claimed in claim 11, wherein the angle of twist is inproportion to the radial dimension of the blade.
 13. A rheometer asclaimed in claim 1 and including means for holding the rotor stationary.